In recent years, in the context of efficient use of energy, there have been demands mainly from transformer manufacturers and the like for an electrical steel sheet with high flux density and low iron loss.
Flux density can be improved by making crystal orientations of the electrical steel sheet in accord with the Goss orientation. JP 4123679 B2, for example, discloses a method of producing a grain-oriented electrical steel sheet having a flux density B8 exceeding 1.97 T.
On the other hand, iron loss properties may be improved by increased purity of the material, high orientation, reduced sheet thickness, addition of Si and Al, and magnetic domain refining (for example, see “Recent progress in soft magnetic steels,” 155th/156th Nishiyama Memorial Technical Seminar, The Iron and Steel Institute of Japan, Feb. 10, 1995). Iron loss properties, however, tend to worsen as the flux density B8 is higher, in general.
It is known, for example, that when the crystal orientations are accorded with the Goss orientation to improve the flux density B8, magnetostatic energy decreases and, therefore, the magnetic domain width widens, causing eddy current loss to rise.
In view of this, as a method of reducing eddy current loss, some techniques have been used to refine magnetic domains by improving film tension (for example, see JP H02-8027 B2) and applying thermal strain.
With the method of improving film tension disclosed in JP '027, however, the strain applied near a elastic region is small, which places a limit on the iron loss reduction effect.
On the other hand, magnetic domain refining by application of thermal strain is performed using plasma flame irradiation, laser irradiation, electron beam irradiation and the like.
For example, JP H07-65106 B2 discloses a method of producing an electrical steel sheet having a reduced iron loss W17/50 of below 0.8 W/kg due to electron beam irradiation. It can be seen from JP '106 that electron beam irradiation is extremely useful for reducing iron loss.
In addition, JP H03-13293 B2 discloses a method of reducing iron loss by applying laser irradiation to a steel sheet.
Meanwhile, it is known that irradiating with a plasma flame, laser, an electron beam and the like increases hysteresis loss, while causing magnetic domain refinement which reduces eddy current loss.
For example, JP 4344264 B2 states that any hardening region caused in a steel sheet through laser irradiation and the like hinders domain wall displacement to increase hysteresis loss. To minimize iron loss, it is thus necessary to reduce eddy current loss while suppressing an increase in hysteresis loss.
To solve the aforementioned problem, some techniques have been proposed to optimize hysteresis loss and eddy current loss in terms of different aspects, and thereby reduce iron loss.
For example, JP '264 discloses a technique to further reduce iron loss by adjusting the laser output and spot diameter ratio to thereby reduce the size of a region, which hardens with laser irradiation, in a direction perpendicular to the laser scanning direction, to 0.6 mm or less, and by suppressing an increase in hysteresis loss due to the irradiation.
In addition, JP 2008-106288 A discloses a technique of reducing iron loss by optimizing the integral value of the compressive residual stress in a rolling direction of a steel sheet in a cross section perpendicular to the sheet width direction to enhance the effect of reducing the eddy current loss.
Furthermore, there has been an increasing demand for reduced transformer noise, as well as high flux density and low iron loss to offer good living conditions. It is believed that the noise of a transformer is primarily caused by stretching movement of the crystal lattice of the iron core, and many studies have shown that reducing single sheet magnetic strain is effective in suppressing the transformer noise (for example, see JP 3500103 B2).
With the conventional methods of reducing iron loss proposed by JP '264 and JP '288, it is possible to reduce either hysteresis loss or eddy current loss, respectively, yet reducing noise at the same time is challenging.
For example, the residual stress distribution illustrated in JP '288 consists of a large, rolling-direction tensile stress near a laser irradiation portion on the steel sheet surface and a relatively large, rolling-direction compressive residual stress produced below in the sheet thickness direction. In this way, when a rolling-direction tensile stress and a rolling-direction compressive stress are concurrently present, the steel sheet tends to deform to release the stresses. Consequently, for transformers fabricated from a combination of such grain-oriented electrical steel sheets, iron cores take such a deformation mode as to release the internal stress upon excitation, in addition to the deformation due to stretching movement of the crystal lattice, resulting in an increase in noise.